It is often necessary, particularly in the petroleum industry, to accurately determine the volume of fluid flowing through a pipe line. In order to ensure their continuing accuracy the flow meters used for this purpose have to be calibrated periodically. The calibration is commonly carried out by a method involving the introduction of a sphere into a calibrating loop connected to the pipe line. Because the diameter of the sphere is such that it contacts the inner surface of the calibrating loop conduit, fluid upstream from the sphere cannot flow past it, thus causing the sphere to travel at the same speed as the fluid. As it begins its travel through the loop the sphere contacts a switch which starts a counter that registers the flow measured by the meter. As the sphere ends its travel it contacts a second switch which stops the counter. By comparing the amount registered by the meter against the known volume of the flow path between the switches the flow meter can thus be calibrated.
Although the basic functioning of pipe line provers is well known, the method by which the sphere is introduced into the prover loop has varied a great deal. An interchange conduit normally connects the inlet and outlet of the loop at points just upstream from the first switch and just downstream from the second switch to allow the sphere to be introduced to the loop inlet and to be recaptured from the loop outlet. The sphere must be introduced in such a manner, however, that the interchange conduit is sealed during travel of the sphere through the loop so as to get an accurate measurement of the fluid flow rate. This has been accomplished in a variety of ways in the past, including the use of a piston or plunger to push a sphere from the interchange conduit into the inlet of the loop while at the same time forcing it through a small diameter portion of the conduit in order to maintain a seal between the sphere and the conduit. Such an arrangement is relatively elaborate and expensive, particularly for large diameter pipe lines, and requires that the sphere and interchange conduit be precisely dimensioned in order to ensure that the sphere can pass through the conduit in sealing engagement. In other arrangements mechanical stops are employed to hold the sphere in place in the interchange conduit, requiring a mechanism which allows the stops to be overcome or moved out of the way when the sphere is introduced into the inlet of the loop. Separate movable mechanical seals have also been employed in the interchange conduit.
It would be desirable to have a simple, inexpensive and relatively maintenance-free system for introducing spheres into the prover loop which assures that a seal will be maintained in the interchange conduit during calibration of the flow meter.